The design of rehabilitative regimens, prosthetic devices, and electronics for the functional neuromuscular stimulation of deficient muscles depends upon precise understanding of the relationship between muscle structure and function. This will be studied in a dynamic system and the specific model to be used is maturing muscle of the feeding apparatus of miniature pigs, as behavior matures from suckling to adult mastication of solid food. The long-term objectives are (1) to interpret the functional significance of changing muscle fiber types during postnatal ontogeny, and (2) to determine the extent to which maturation of muscle is entirely age-dependent or has an inducible component that is sensitive to changes in jaw loading. Postnatal developmental trajectories of function and structure will be documented for two jaw muscles. One-half of the sample will progress ad libitum from liquids to chewing of solid foods, with the remainder maintained on a nutritionally equivalent but liquified diet. Muscle function will be determined by monitoring electromyographic activity during normal feeding behavior in pigs from 2 to 14 weeks of age. Enzyme histochemistry will be used to document ontogenetic changes in fiber type composition in 2, 4, 8, and 14 week old pigs. Fiber types will be verified using immunocytochemistry and electrophoresis to characterize variation in order to track the sequential replacement of embryonic, neonatal, and adult myosin isoforms.